Method of making cylindrically curved printing plates



July 1, 1958 KIRKPATRICK ETAL 2,341,083

METHOD OF MAK YLINDRICALLY CURVED PRINTING PLATES F1196 Aug- 2, 1954 2Sheets-Sheet 1 FIG.4.

JAMES $.KIRKPATRICK BY PETER ZYLSTRA JR. M.

A? ATTORNEYS United States Patent METHOD OF MAKING CYLINDRICALLY CURVEDPRINTING PLATES James 'S. Kirkpatrick, Detroit, and Peter Zylstra, Jl'.,Plymouth, Mich., assignors to Brooks & Perkins, Inc., Detroit, Mich., acorporation of Delaware Application August 2, 1954, Serial No. 447,14213 Claims. (Cl. 101-4011 The present invention relates to a method ofmaking cylindrically curved printing plates.

It is an object ,of the present invention to provide a method forforming printing plates of magnesium which comprises bending a fiatmagnesium printing plate into cylindrical configuration with theprinting surface on the convex side thereof by heating the plate to atemperature above 300 degrees Fahrenheit, compressing the plate with itsprinting side outermost against a cylindrical rigid forming member,covering the printing surface of the plate with a thin covering ofaluminum, applying the forming pressure to the plate through a yieldablematerial, trapping the yieldable material against the printing surfaceof the plate, applying an essentially hydraulic pressure through thetrapped yieldable material of an amount dependent on plate temperature,and finally removing the aluminum from the cylindrically curvedmagnesium printing plate by treatment in a caustic bath.

Other objects and features of the invention will become apparent as thedescription proceeds, especially when taken in conjunction with theaccompanying drawings, wherein:

Figure l is a perspective view of a cylindrically curved printing plateconstructed in accordance with the present invention.

Figure 2 is afragmentary enlarged sectional view of a portion of theprinting plate shown in Figure 1.

Figure 3 is a fragmentary sectional view of a portion of theprintingplate upon removal from the press showing the printing surface coveredwith a covering or coating of aluminum.

Figure 4 is a diagrammaticsectional view showing the printing plate inthe forming press.

Figure 5 is a chart showing permissible temperature and pressure limitsin forming the curved plate.

In accordance with the present invention printing plates formed ofmagnesium or an alloy composed mainly of magnesium are producedbyconventional practices in the form of a flat plate. Thereafter, theflat plate 1is curved to a true cylindrical configuration with theprinting surface of the plate exposed at the convex side thereof.

. Printing plates formed of magnesium offer many advantages over platespreviously used in theprinting industry. One of the important advantagesof magnesium printing plates is the extreme lightness of such plates ascompared to materials previously employed. Modern high speed pressesproduce relatively large centrifugal forces which of course aredependent upon the weight of the printing plates. Obviously, the use ofextremely light magnesium printing plates reduces the centrifugal forcesand will accordingly permit faster press operation.

In order to successfully employ magnesium printing plates as disclosedabove, it is essential that the plates in their final form have asubstantially true cylindrical curvature extending from edge to edgethereof. Previous methods of bending flat plates to generallycylindrical configuration have in general proved unsatisfactory. It hasbeen found however, that when flat magnesium printing plates are curvedto cylindrical configuration against a rigid cylindrical forming memberwhile heated to temperatures above 300' degrees Fahrenheit by theapplication of essentially hydraulic pressure dependent upon platetemprinting surface.

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perature, but preferably of at least 1000 pounds per square inch,magnesium plates retain the exact cylindrical curvature imparted theretowithout spring back and accordingly, such plates may be used in theirentirety in rotary presses.

It has been found that the magnesium printing plates may be curved fromtheir initial flat shape to the true cylindrical configuration requiredthroughout a substantial range of pressures and temperatures. Figure 5is a chart illustrating these ranges. It will be observed that theminimum temperature under which the fiat plates may be curved tocylindrical configuration is 300 degrees Fahrenheit since below thistemperature the plates have a tendency to crack. The maximum temperatureis shown as slightly in excess of 800 degrees since above thistemperature the plate becomes so soft that the forming operation resultsin damage to the printing characters on the plate.

Within these temperature limits the necessary pressure to produce thepermanent deformation of the plate to true cylindrical configurationwithout dimensional distortion or distortion of the printing charactersmay be varied substantially. Thus for example, at 300 degrees pressurebetween 3,000 and 18,000 pounds per square inch may be employed. .As thetemperature increases the maximum pressure which may be applied to theplate falls off rather rapidly, approaching zero as the platetemperature exceeds 800 degrees Fahrenheit. 7 The actual pressure andtemperature of course will normally depend upon equipment available andwhere means for providing extremely high pressures is not available, theoperation will of course be carried out at higher temperatures.

It is pointed out that not only must the temperature and pressure berelated as to minimum values, but also as to maximum values so as toprevent growth of the plate during the forming operation, with resultantlack of registration in the printing operation and also to avoidmodification of the printing characters on the plate.

In practice, excellent results have been obtained when the plates havebeen formed at temperatures between 450 and 650 degrees Fahrenheit andat pressures in excess of 1,000 pounds per square inch.

.In order toapply the hydraulic pressure required to cause the plates toretain the shape imparted to them in the press, a yieldablesubstantially incompressible material such for example as a suitablerubber compound is trapped against the printing surface of the plate.Inasmuch as this rubber material is substantially incompressible theforces developed in the press are essentially hydraulic in character.

It has been found however, that the pressures required in thisoperation, in conjunction with the relatively high temperatures causedeterioration of the rubber with the result that particles of rubberbecome separated from the yieldable material and remain in concaveportions of the Even when the rubber compound is new, relatively finecrevices or depressions in the printing surface have the rubber compoundembedded therein after the pressing operation.

Accordingly, to make the operation as above described commerciallypracticable, it has been found that the trapping or adherence ofparticles of rubber to the printing surface of the plate may be avoidedby providing a thin covering of aluminum between the printing surfaceand the rubber compound prior to the application of pressure through theyieldable material. The application of pressure of course causes thethin covering of aluminum to conform to the configuration of theprinting surface and in some cases, particles of rubber may adhere orbecome trapped in concave elements of the aluminum covered surface.Following the pressing opperation however, the cylindrically curvedmagnesium printing plate is immersed in a caustic bath and the aluminumeffectively removed thereby without affecting the magnesium.

In Figure 1 there is shown a cyliudrically curved printing plate havinga convex cylindrical surface 12 provided with a printing surface, suchsurface being diagrammatically indicated at 14. i

In Figure 2 there is shown an enlarged section of the printing plate andthe relatively raised and depressed portions 16 and 18 of the printingsurface are indicated.

In Figure 4 there is illustrated a press useful in carrying out themethod of the present invention.

The press includes a bed plate 20 on which is mounted a lower platen 22which carries the cylindrically shaped forming block 24. The pressincludes a downwardly movable upper ram plate 26 having a convex rubberretainer 28 secured thereto provided with a downwardly open recess 30.Preferably, the recess is rectangular in shape for the purpose ofapplying pressure to a printing plate in a particular manner. Within therecess 30 is provided a plurality of rubber slabs indicated generally at32. The slabs 32 are of a suitable rubber compound adapted to resistdeterioration when subjected to the temperatures employed in theprocess.

In carrying out this operation a fiat printing plate 10 formed ofmagnesium and having its upper surface a printing surface and covered bythin aluminum is placed on the block 24. Thereafter, the upper ram plate26 is lowered, bending the printing plate to conform to the uppercylindrically convex surface of the block 24. At some point in itsdownward movement the lower edge of the recess 30 passes over the edgesof the platen 22, thereby effectively trapping the rubber slabs withinthe recess 30. Further downward movement of the upper ram plate 26results in the development of essentially hydraulic pressure within therubber material so that every element of the magnesium plate issubjected to the relatively high pressure referred to above. It has beenfound that excessive pressure tends to elongate the printing plate andmay in some instances tend to reduce the sharpness or otherwise modifythe printing characters on the plate.

Referring now to Figure 3 there is illustrated a section of the curvedprinting plate after its removal from the press. As illustrated in thisfigure the printing surface thereof is covered with a thin covering ofaluminum indicated at 40.

Preferably, the rubber slabs 32 are so shaped that after the plate isbent to the configuration illustrated in Figure 4, further downwardmovement of the ram plate 26 results in building up of pressure from thecentral portion of the printing plate progressively outwardly from thecenter of the plate toward the ends, as seen in this figure.

The aluminum covering, which protects the magnesium surface fromadherence or trapping of rubber particles, may be applied in any one ofseveral ways. In the first place, the aluminum coating may convenientlybe applied in the form of a thin aluminum foil which is merely placedover the printing surface before the application of pressure through therubber slabs 32. The thickness of the aluminum foil may be between .0005inch and .010 inch. In practice, the thinner foil may. be employed whenthe rubber is in new condition, and thicker foil may be employed as therubber ages and is subject to some deterioration. Above .010 inch it isfound that the foil does not conform sufficiently easily to the elementsof the printing surface.

Alternatively, the aluminum covering may be applied as a spray ofaluminum powder, or as a variation, the aluminum covering may be appliedas an atomized melted spray.

In any case, following the operation the aluminum covering together withany particles of rubber adhered or trapped thereon, is removed bytreatment of the formed printing plate in a caustic solution. Excellentresults have been obtained by immersing the curved printing plate in acaustic bath including 10 percent by weight ofsodium hydroxide. However,the exact strength of the caustic solution is not particularly criticaland the caustic bath may contain from 1 to 20% by weight of caustic. Thestrength of the caustic results primarily in a difference intimerequired in the treatment to remove the aluminum covering. Insteadof sodium hydroxide, any suitable caustic'efiective to dissolve orpartly dissolve and remove the aluminum covering is used.

To facilitate removal of the aluminum covering and any particles ofrubber adhered thereby or embedded therein, the printing plate may begivena light brush treatment to accelerate cleaning of the magnesiumprinting surface. 7

When the method as outlined herein is followed, it is found that theprinting plate retains its exact cylindrical configuration after removalfrom the press, and it is found further that the aluminum covering overthe previously produced printing surface protects this surface fromcontamination by particles of rubber and at the same time avoids anyappreciable effect on theaccuracy and detail of the printing surface.

While the printing plate herein is referred to as a magnesium plate, itwill be understood that the present invention includes within its scopemagnesium alloys including relatively high percentages of magnesium suchfor example as 95% magnesium as a minimum. It is specifically meant toinclude plates which may contain a relatively minor percent of othermaterials, such for example as zinc. The fact that the plate isessentially composed of magnesium however, gives rise to the possibilityof producing accurately formed cylindrical plates without spring-backwhich retain the exact cylindrical form imparted thereto in the press.Moreover, the use of the aluminum covering over the printing surfacepermits the application to the hot plate of pressure through yieldablerubber materials.

The drawings and the foregoing specification constitute a description ofthe improved method'of making cylindrlcally curved printing plates insuch full, clear, concise and exact terms as to enable any personskilled in the art to practice the invention, the scope of which isindicated by the appended claims. 1

' What we claim as our invention is:

1. The method of making cylindrically curved printing plates whichcomprises providing a flat magnesium plate with a printing surface atone side thereof, engaging the other side of the plate with a rigidconvex cylindrical forming member, pressing a yieldablerubber materialagainst the printing surface of the plate to cause the plate to conformto the forming member, maintaining the'plate at a temperature of between300 and 800 degrees Fahrenheit during the application of pressure, andtrapping the rubber material while increasing the applied pressure andthus applying an essentially hydraulic pressure through the rubbermaterial, the pressure being selected in accordance with the temperatureof the plate and having limits in accordance with the following table:

2. The method of making cylindrically curved printing plates whichcomprises providing a flat magnesium plate with a printing surface atone side thereof, engaging the other side of the plate with a rigidconvex cylindrical forming member, pressing a yieldable rubber materialagainst the printing surface of the plate to cause the plate to conformto the forming member, maintaining the plate at selected temperaturesduring the application of pressure, and trapping the rubber materialwhile increasing the applied pressure and thus applying an essentiallyhydraulic pressure through the rubber material, the pressure beingselected in accordance with the temperature of the plate and havinglimits falling within the shaded area of the chart of Figure 5.

3. The method of making cylindrically .curved printing plates whichcomprises providing a flat magnesium plate with a printing surface atone side thereof, engaging the other side of the plate with a rigidconvex cylindrical forming member, covering the printing surface with athin covering of alumium, pressing a yieldable substantiallyincompressible rubber material against the aluminum covered printingsurface to cause the plate to conform to the forming member, maintainingthe plate at a temperature of between 300 and 800 degrees Fahrenheitduring the application of pressure, trapping the yieldable materialagainst the aluminum covered printing face of the plate and applying anessentially hydraulic pressure through the yieldable material, andfinally, removing the aluminum covering from the printing surface bytreatment with a caustic bath.

4. The method of claim 3 in which the aluminum covering is between .0005and .010 inch in thickness.

5. The method of claim 4 in which the aluminum covering is in the formof a foil interposed between the plate and yieldable material.

6. The method of claim 4 in which the aluminum covering is in the formof comminuted particles interposed between the plate and yieldablematerial.

7. The method of claim 4 in which the aluminum covering is applied inthe'form of a powder spray interposed between the plate and yieldablematerial.

8. The method of claim 4 in which the aluminum covering is applied inthe form of atomized melted spray interposed between the plate andyieldable material.

9. The method of claim 3 in which the step of treatment in a causticbath comprises immersion in a caustic solution of sodium hydroxidebetween 1% and 20% by weight of the caustic.

10. The method of claim 3 in which the step of treatment in a causticbath comprises immersion in a caustic solution of sodium hydroxide ofabout by weight of the caustic.

11. The method of claim 9 which comprises brushing the printing surfaceof the plate to facilitate removal of aluminum and rubber particles.

12. The method of making cylindrically curved printing plates whichcomprises providing a flat magnesium plate with a printing surface atone side thereof, engaging the other side of the plate with a rigidconvex cylindrical forming member, covering the printing surface with athin covering of aluminum, pressing a yieldable substantiallyincompressible rubber material against the aluminum covered printingsurface to cause the plate to conform to the forming member, maintainingthe plate at a temperature of between 450 and 650 degrees Fahrenheitduring the application of pressure, trapping the yieldable materialagainst the aluminum covered printing face of the plate and applying apressure whose value is within the area determined by a pressure ofbetween 1450 and 9700 pounds per square inch when the temperature is 450degrees Fahrenheit, and between 300 .and 3750 pounds per square inchwhen the temperature is 650 degrees Fahrenheit, and finally removing thealuminum covering from the printing surface by treatment with a causticbath.

13. The method of making cylindrically curved printing plates whichcomprises providing a fiat magnesium plate with a printing surface atone side thereof, engaging the other side of the plate with a rigidconvex cylindrical forming member, covering the printing surface with athin covering of aluminum, pressing a yieldable substantiallyincompressible rubber material against the aluminum covered printingsurface to cause the plate to conform to the forming member, maintainingthe plate at a temperature of between 300 and 800 degrees Fahrenheitduring the application of pressure, trapping the yieldable materialagainst the aluminum covered printing face of the plate and applying anessentially hydraulic pressure through the rubber material, the pressurebeing selected in accordance with the temperature of the plate andhaving limits of between 3200 and 18,500 pounds per square inch when theplate has a temperature of 300 degrees Fahrenheit, the pressure limitsdecreasing toward zero as the temperature of the plate exceeds 800degrees Fahrenheit, and finally removing the aluminum covering from theprinting surface by treatment with a caustic bath.

References Cited in the file of this patent UNITED STATES PATENTS1,276,532 Hubbard Aug. 20, 1918 1,688,648 Novotny Oct. 23, 19282,486,130 Dietrich et al Oct. 25, 1949 OTHER REFERENCES Gray: Formingmagnesium alloy, pub. in April 21, 1947, issue of Steel, pages 91 and126 to 134.

Harvey et al.: Magnesium sheet easily formed pub. in May 25, 1944, issueof American Machinist, pages -108.

Ashburn: How to work magnesium alloys, pub. in November 7, 1946, issueof American Machinist, pages 117-132.

Flader et al.: Modern Photoengraving, pub. 1948, Modern PhotoengravingPublishers, Chicago, Cincinnati. Page 185.

